Radio communications systems use transmitters to transmit modulated radio frequency (RF) signals and receivers to process the received RF signals. Typical processing of received RF signals involve converting relatively high-frequency incoming signals to a relatively low-frequency signal, which is then demodulated to extract useful information from the originally transmitted signal. The frequency conversion process usually introduces low-frequency distortion, most commonly in the form of a direct current offset (DC offset) signal, into the converted signal. The DC offset is especially problematic with amplifiers and analog-to-digital converters in the radio receiver because an excessively large DC offset can result in the amplifiers distorting the demodulated signal while compressing the useful range of the analog-to-digital converters and reducing its effective resolution.
The majority of radio receivers today use a superheterodyne architecture, which can perform multiple frequency conversion to the input signal before it is finally demodulated. Typically, the demodulation is performed at an intermediate frequency (IF), which is low enough for easy amplification but remains high enough to accommodate the modulated signal. Due to the use of potentially multiple frequency conversions and high intermediate frequencies, the removal of most of the DC offsets in superheterodyne architecture receivers can be readily achieved through the use of high pass filters.
In another type of radio receivers, known as direct-conversion receivers, the received signal is immediately converted down to a very low frequency (the baseband frequency) without going through any intermediate frequencies. Direct-conversion receivers are gaining popularity because they do not require any intermediate filters, mixers and amplifiers as does the superheterodyne receiver, therefore resulting in a simpler and less expensive radio receiver. The direct-conversion receiver can usually be integrated onto a single integrated circuit, mainly from their use of low-pass filters that are easily fabricated in monolithic form.
However, the actual frequency downconversion process used in direct-conversion receivers can introduce a significant amount of DC offset. Additionally, due to the received signal being centered around the zero frequency, the commonly used method to remove DC offset, high-pass filters, is not an effective solution. The use of high-pass filters to remove DC offset result in either loss or distortion of a significant amount of the downconverted signal. This results in an overall reduction in the available bandwidth. The filters can also introduce phase errors into the data signal. The reduction in available bandwidth and introduction of phase errors places limits on data rates and decreases the noise tolerance of the radio receiver.
Another proposed solution takes advantage of idle receive times in systems with intermittent transmission in order to store the DC offset present in the absence of the input signal and then subtract the stored value when receiving the input signal. This solution requires sufficient idle time in order to transmit the DC offset value. This solution also requires that the DC offset be measured early in the radio receiver's receive path and then the DC offset be removed later in the receive path. This is potentially problematic if the received signal has already been amplified and/or converted by an analog-to-digital converter and has been distorted prior to the DC offset removal. Additionally, in a high data-rate receiver application, sufficient idle time is simply not available to store the DC offset in the received signal. A need has therefore arisen for a method for removing the DC offset from a communications system which does not introduce phase errors or noise into the system and maximizes available bandwidth.